The present invention relates to magnetic thin film structures, and in particular to a three-part etching process for selectively etching exposed magnetic layers of a magnetic random access memory (MRAM) stack.
Ion milling has been essentially the only method available for creating fine patterns, e.g., submicron patterns, in magnetic thin film structures. Because of the lack of volatile compounds for ferrous metals other than carbonyl, reactive-ion etching (RIE) has not been a viable technique for patterning thin magnetic films; a RIE process based on carbonyl chemistry has not yet been developed. Thus, a chemical etching technique for patterning magnetic thin films based on Fe, Co and Ni is attractive. The MRAM structure represents a complex multilayer system which includes numerous magnetic thin film layers. A typical MRAM structure is shown in FIG. 1. Specifically, the thin film structure shown in FIG. 1 comprises Si substrate 10, SiOx layer 12, a 150 xc3x85 Ti layer 14, Ni81Fe19 (40 xc3x85) layer 16, Ir20 Mn80 (120 xc3x85) layer 18, Co90Fel10 (20 xc3x85) layer 20, Al2O3 (10 xc3x85) layer 22, Ni81Fe19 (40 xc3x85) layer 24 and Ti (100 xc3x85) layer 26. In this prior art magnetic structure, Al2O3 layer 22 serves as a tunnel barrier between the top magnetic film layer, i.e., Ni81Fe19 layer 24, and antiferromagnetic layer 18 and magnetic layers 16 and 20 which are present beneath the tunnel barrier layer. Layer 26 is a passivating layer that prevents moisture, air or other contaminants from entering into the structure, while layer 14 is an adhesion layer.
As can be seen, the magnetic films of the MRAM structure illustrated in FIG. 1 are quite thin. Patterning of the MRAM structure of FIG. 1 is typically carried out in the prior art by first applying a mask to the MRAM structure and patterning the mask by lithography (exposure and development). FIG. 2 shows the structure after these steps wherein reference numeral 28 represents the patterned mask. The pattern is then transferred to the MRAM structure by first removing the top 100 xc3x85 Ti film of the MRAM structure by either RIE or wet etching, and then a standard aqueous acid solution such as sulfuric and nitric acid is employed to etch the exposed free-magnetic Ni81Fe19 (40 xc3x85) layer. Although acid etchants are capable of etching through the exposed top magnetic layer of the structure, acid etchants are not selective for removing just that exposed magnetic layer. Instead, when acid etchants are employed, they also etch the underlying alumina tunnel barrier layer, the Co90Fe10 layer, and the Mn in the Ir20Mn80 layer of the magnetic thin film stack providing the structure shown in FIG. 3.
Despite being capable of etching numerous magnetic layers in the MRAM structure, the use of prior art aqueous acid solutions causes Galvanic-coupling-accelerated dissolution of the Co90Fe10 (20 xc3x85) layer in the film region under the mask which is unacceptable for many applications. An ideal situation would be to etch through the top barrier layer and the top exposed magnetic layer, i.e., layer 24, stopping at the thin Al2O3 layer, thereby leaving the underlying layers, i.e., layers 16, 18, and 20, unetched.
To date, applicants are unaware of any etching process which selectively etches a magnetic thin film structure so as to stop on the tunnel barrier layer present in the structure. There is thus a need for developing an etching process which is capable of selectively etching the magnetic thin film structure so as to provide a patterned structure wherein the pattern is not formed in the tunnel barrier layer or the magnetic layers that are located beneath the tunnel barrier layer of the structure. Such an etching process would be beneficial since it would prevent unwanted Galvanic corrosion of the inner magnetic layers, while being able to pattern the top barrier layer and the top magnetic film layer of the structure.
The present invention is directed to a method of selectively patterning the top magnetic film layers of a magnetic structure stopping on the tunnel barrier layer in which the various etching processes employed do not adversely damage the tunnel barrier layer and the magnetic thin film layers that are present beneath the tunnel barrier layer. The aforementioned object is achieved by utilizing a three-part selective etching process. Specifically, the above object is obtainable utilizing processing steps that include:
(a) providing a magnetic structure which includes at least one bottom magnetic film layer and at least one top magnetic film layer, wherein said top and bottom magnetic film layers are separated by a tunnel barrier layer, and said at least one top magnetic film layer having a passivating layer formed thereon, said passivating layer including a surface oxide region formed therein;
(b) forming a patterned resist on said passivating layer wherein a portion of said passivating layer is exposed;
(c) removing said surface oxide layer from said exposed portion of said passivating layer by reactive-ion etching;
(d) selectively etching said exposed portion of said passivating layer by a wet etch process which includes an etchant solution comprising an organic acid, a fluoride salt and an inhibitor which prevents pitting and dissolution of said at least one top magnetic film layer whereby a portion of said at least one top magnetic film layer is exposed; and
(e) selectively etching said exposed portion of said at least one top magnetic film layer by a wet etch process which includes a dicarboxylic acid aqueous etchant solution stopping on said tunnel barrier layer.
In one alternative embodiment of the present invention, a sulfur-containing compound is added to the three-component etchant solution of step (d); and, in another alternative embodiment of the present invention, the three-component etchant solution of step (d) is replaced by a SF6-RIE plasma etching process.